1. Field of the Invention
The present invention relates to silica glass including titania, and particularly relates to titania-silica glass suitably used as a photomask material for extreme ultraviolet ray lithography (hereafter referred to as EUV lithography) in a manufacturing process for a semiconductor, a liquid crystal, etc.
2. Description of the Related Art
In high integration of a semiconductor integrated circuit in recent years, an ultra-fine processing technology plays the most important role. As the semiconductor integrated circuit is subjected to a finer process, development of optical lithography technologies is also progressing. The EUV lithography using an EUV ray for an exposure light source attracts attention as one of them.
Since this EUV ray has a wavelength of 13.5 nm or less and is absorbed by any material, a lithography system of a catoptric system is employed.
In the EUV lithography, since a photomask substrate is irradiated with a high power laser, it requires a thermal stability in the order of subnanometers.
Therefore, as a material of the photomask substrate, the glass is needed which has a thermal expansion coefficient lower than that of the silica glass used for the conventional photolithography.
As an example of such glass having a low thermal expansion coefficient, titania-silica glass is known.
For example, Published Japanese Translation (of PCT Application) No. 2003-505876 discloses that the silica glass is used which includes 5 to 10% by weight of titania and has a thermal expansion coefficient within a range of ±10 ppb at 20° C.
Generally, such titania-silica glass as described above is manufactured in such a way that a mixture of a silica precursor and a titania precursor in a steam form is hydrolyzed by a burner in an oxy-hydrogen flame, and titania-silica glass particles are deposited and melted at the same time.
However, in the above-mentioned manufacture method, a glass body is directly formed by hydrolyzing the titania precursor and the silica precursor. Thus, a titanium ion of titania which is athermal is tetravalent (Ti4+) in an oxidization atmosphere, and a titanium atom is a constituent member of a glass network where a silicon atom is replaced with the titanium atom. The glass which has such a network tends to be influenced by temperature, and its thermal expansion coefficient is larger at a higher temperature.
Thus, in the titania-silica glass of a two-part system, since its thermal expansion coefficient tends to be influenced by a titania content and temperature, it is difficult to obtain glass having a low thermal expansion coefficient which is excellent in homogeneity within a field and has excellent stability.